The majority of present day light emitting diodes (LEDs) are built on planar technology. The PN-junction is constructed as a plurality of layers on a substrate giving a device with an essentially horizontal orientation. The light-producing re-combination takes place in one of the layers. As the layer has a refractive index that is substantially higher than the refractive index of the air, a substantial portion of generated light will be reflected in the layer and not contribute to the effective luminescence of the device. In fact the layer will act as a waveguide in the horizontal plane of the LED. Measures have been suggested to mitigate the effects of the light of LED being trapped in the device and to efficiently extract the light out of the semiconductor layers. Such measures include modifying the surface in order to provide portions with varying angles to the horizontal plane. A similar approach is suggested in EP1369935, wherein nanosized particles are provided in the LED device to scatter light or alternatively absorb light and generate light of a different wavelength. In addition the planar technology imposes constrains in terms of miniaturization and choices of suitable materials, which will be further described below.
The development of nanoscale technology and in particular the ability to produce nanowires have opened up possibilities of designing structures and combining materials in ways not possible in planar technology. One basis for this development is that the 1D properties of a nanowire makes it possible to overcome the requirement of lattice matching between different materials in a device made with planar technology. It has been shown and utilized that nanowires of for example InP can be grown on InAs or Si without defects. In US Published Application 2004/0075464 by Samuelson et al a plurality of devices based on nanowire structures are disclosed, for example nanowire LEDs. These LEDs have an internal heterostructure giving quantum confinement effects.
US Published Application 2003/0168964 teaches an assembly of a plurality of nanowires acting as LEDs mounted in groups between a conductive transparent substrate at the lower end of the nanowires and a transparent cover substrate at the top end, each individual nanowire having a structure of P-type, N-type and light emitting layer. The nanowires are said to be arranged to emit light through the conductive transparent substrate.
Other nanowire LED have previously been reported. Hiruma et al. fabricated vertical GaAs nanowire pn LEDs. The nanowires were embedded in a SOG and covered with an Au/Ge/Ni top contact, as described in “GaAs p-n junction formed in quantum crystals” by Haraguchi et al., Appl. Phys. Lett. 60 (6) 1992. These devices showed room temperature electro luminescence. GaN based nanowire LEDs have also been fabricated as described in “Core/Multishell Nanowire Heterostructure as Multicolor, High-Efficiency Light-Emitting Diodes” by F. Qian et al., recently published in the journal Nano Letters.